Novel Thallium and Bismuth based materials provide advantages

As an alternative to HgCdTe, a number of III-V semiconductor systems have been investigated due to a potential for higher uniformity, yield, and mechanical strength. Some of the material system researched for uncooled mid and far-infrared detectors include: InSb, InAsSb, InTlSb, InSbBi, and InTlAsSb. Also, much work recently has been devoted towards type-II InAs/GaSb superlattices.

InTlSb Detectors

As an alternative to the InAsSb system, InTlSb is also a potential material for λ> 8 μm detectors. By incorporating Tl into an InSb lattice, the material can go from a semi-conducting to a semimetallic phase (Eg<0). Unfortunately, due to a large miscibility gap which limits the Tl incorporation to about 15 %, it was almost impossible to incorporate any Tl experimentally.

Spectral response from InTlSb photodetectors.

The first epitaxial growth of InTlSb on GaAs substrates using LP-MOCVD was done at the Center for Quantum Devices. Figure b shows the spectral response of In1- xTlxSb photodetectors.

InSbBi Detectors

Besides adding As or Tl to an InSb lattice, it is also possible to substitute Bi. As a result, a rapid reduction of the bandgap is expected. Again however, a large miscibility gap exists which limits Bi incorporation.

Spectral response from InSbBi photodetectors.

Recently, the first successful growth of InSbBi epitaxial la yers with a substantial amount of Bi (~5 %) by LP-MOCVD was reported. Growth was performed both on InSb and GaAs substrates. Figure a shows the variation of the material photoresponse as a function of composition on GaAs substrate at 77 K. Fabricating InSb0.95Bi0.05 into a photoconductive detector yielded room temperature operation with a cutoff out to 12 μm.

InTlAsSb Detectors

In order to exceed the limitations imposed by the InTlSb miscibility gap, arsenic was added to form a quaternary material with an extended photoresponse cutoff wavelength. This work led to the first successful demonstration of InTlAsSb on GaAs substrates with a cutoff wavelength up to 15 μm at room temperature. These measurements were the first observation of room temperature band edge photoresponse at such long wavelengths from III-V alloys.